Horus Heresy/YMMV

And the Fandom Rejoiced: OLLANIUS PIUS LIVES!
Alternate Character Interpretation: The Horus Heresy novels tweak a lot of the various background details from the tabletop.
- Magnus the Red is known in the game as being a cyclops; in addition to his mighty psychic powers, his mutated genome caused him to only bear one eye, which was part of the reason why he was accepted by the planet of peaceful mutants he was "born" on. In Horus Heresy, he isn't mutated at all, he's just lost an eye.
- The Emperor is portrayed as either a moron or extreme Jerkass (or a bit of both) who basically directly influenced the Traitor Legions to turn against him for understandable reasons.
  - Or a man with too little time, who had to either choose to coddle his adult long lost children or hope they were mature enough to deal with real life without daddy holding their hand?
- The Space Wolves are regarded as savage, brutish, even unworthy of being Space Marines by their fellows -- in the 41st millenium, they are actually one of the nicer chapters.
- The Word Bearers, in "The First Heretic", are revealed to have a "quirk" in their gene-seed that induces extremely strong loyalty in them. Several of the characters who are aware of this openly worry about how much of their deep bond to Lorgar is natural and how much is influenced by this gene-flaw that fills them with an intense need for someone/thing to have faith in.
- The nature of the Second and Eleventh Primarchs and their Legions is left mysterious in the game in order to encourage players to create their own Space Marine chapters. In the Horus Heresy novels, it's stated several times that these two Primarchs and their Legions were wiped out and Unpersoned by the Emperor and his other Legions for something, well before Horus was corrupted by Chaos. The Legions themselves may have been made into Ultramarines.
  - This is left ambiguous, if I recall correctly.
    - To teh degree of what exactly happened sure, but it seems quite clear there was some kind of "heresy" with them.
- In "The First Heretic", a daemon allows some emissaries from the Word Bearers to see through time and behold the embryonic Primarchs just before they are scattered across the galaxy. Not only does it claim that the Emperor actually bartered with Chaos to acquire the knowledge he needed to make the Primarchs and then sought to cheat them, it is actually direct influence from these time-traveling Space Marines that causes them to be scattered.
  - Didnt Horus get to do the same in one of the early books?
- In a recent interview, Dan Abnett says they often have to chose between conflicting versions of the same tale when they write it up for inclusion in this series, and that they think of these books as what actually happened, and the versions of the tales we know from earlier works being the versions as filtered through 10,000 years of oral history repetition between the Heresy and Warhammer40k proper.
Badass Normal
- Ollanius Pius anyone?
Complete Monster - Many of them. Most surprisingly of all in light of their relative 'goodness' in 40k, the Space Wolves are this intentionally as the Emperor's executioners.
Designated Hero - Even before the actual heresy and before any Space Marine had fallen to chaos, many of the actions undertaken by the primarchs and the expedition fleets were... questionable, at best. Fulgrims and Ferrus Manus's ruthless extermination of the Diasporex, a democratic confederacy where humans and aliens lived in peace together and only wanted to be left alone, simply for the humans co-operation with xenos and refusal to join the Imperium, is propably the best example.
Demonic Possession
- Spear, in Nemesis ? Not sure as he's not so much possessed as he's wearing a daemon as his skin.
Hilarious in Hindsight - That quote from the scholar Karkasy in the first Horus Heresy book: "Most vigorous of all was the Imperial Creed that insisted humanity adopt the Emperor as a divine being. A God-Emperor of Mankind. The idea was ludicrous and, officially, heretical. The Emperor had always refused such adoration in the most stringent terms, denying his apotheosis. Some said it would only happen after his death, and as he was functionally immortal, that tended to cap the argument.".
Magnificent Bastard - Horus. Again.
- Don't forget Lion E'Jonson, he loves his Xanatos gambits secrets but he is the primarch of the Dark Angels.
  - Lorgar and his Legion tend to take the cake, given they set everything rolling.
    - The Alpha Legion operative in the Age of Darkness story 'Liar's Due' has this as his hat. He manages to make a loyalist world self destruct/declare for Horus with a single broadcast and their own prejudices.
    - The demon in Prospero Burns. It planted a spy in the Space Wolves that it knew the Wolves would find and believe was sent by the Thousand Sons, which in turn guaranteed that when Magnus the Red tried to warn the Emperor of Horus' impending betrayal via sorcery, he would not only be disbelieved, but treated as a traitor. As a direct result, the Space Wolves all but annihilated the Thousand Sons, and Magnus turned in desperation to Tzeentch to save what remained.
    - Not only that, but the above plan was considered only a partial victory. The demon (possibly Tzeentch/one of his agents) had intended for the Space Wolves/Russ and the Thousand Sons/Magnus to destroy one another completely, as they represented the most powerful magical and physical barriers (respectively) to Horus' impending heresy. Instead, the Space Wolves were only bloodied, but the Thousand Sons survived and turned to Chaos.
    - And now, as of Age of Darkness's last story, it appears that Lion El'Johnson may not be as deceptive as first appears. That honour would go to... Roboute Guilliman.
Moral Event Horizon - Pretty much what the series is about.
Motive Decay - Horus suffers a bad case of "This is your motive on Chaos".
Pay Evil Unto Evil
- Multiple times the Horus Heresy paints the Emperor as, bluntly, an arrogant prick who held a great deal of responsibility for his sons falling and the heresy occurring in the first place. He waits a century before censuring Lorgar for establishing the Imperial Cult on conquered worlds, while simultaneously humiliating the Primarch and the entire Legion and destroying the innocent inhabitants of a world to make a point. He refuses to explain to Horus why he is leaving the Great Crusade, despite their closeness and the obvious damage the lack of trust did to his most favoured son. Magnus the Red arguably fared worst of all, as the Emperor specifically sends Russ and the Space Wolves, a Primarch and Legion who loathe Magnus and the Thousand Sons to their core to bring Magnus back to Terra, making it almost child's play for Horus to change the order to one of extermination
The Starscream: Age of Darkness reveals that Guilliman is training his forces to fight all legions, both loyal and traitor, so he can replace the Imperium with his own empire.
Villain Sue: Spear from Nemesis.
- Justified based upon the number of enhancements he was given and that he was created purely to kill The Emperor.
What an Idiot!: The Emperor's entire approach to knowledge of Chaos, knowledge of his plans, certain actions taken towards his sons which almost certainly contributed directly to their corruption and/or betrayal. For a being of such towering intellect, the Emperor seems to have made it his goal during his later life to take the Idiot Ball and set a new scoring record.